Structure characteristics of cylindrical, square and soft pack lithium batteries
At present, there are three main packaging forms of lithium battery, that is, cylinder, square and soft package. Different packaging structures mean different characteristics, and they have their own advantages and disadvantages.
1、 Cylindrical lithium battery
Cylindrical lithium battery refers to cylindrical lithium battery. The earliest cylindrical lithium battery was 18650 lithium battery invented by Sony Company of Japan in 1992. Because 18650 cylindrical lithium battery has a long history, its market penetration is very high. Cylindrical lithium battery adopts quite mature winding technology, with high automation, stable product transmission quality and relatively low cost. There are many types of cylindrical lithium batteries, such as 14650, 17490, 18650, 21700, 26650, etc. Cylindrical lithium battery is more popular in Japan and South Korea, and there are quite large-scale enterprises in China to produce cylindrical lithium battery.
With the further expansion of the electric vehicle market and the continuous improvement of the requirements for the endurance mileage, the vehicle enterprises put forward higher requirements for the power battery in terms of energy density, manufacturing cost, cycle life and product additional attributes. Under the premise that the field of raw materials has not yet achieved a great breakthrough, it is an exploratory direction to appropriately increase the volume of cylindrical battery to obtain more battery capacity. At present, Tesla has started the large-scale production of 21700 batteries and plans to use them on Tesla Model 3. If they pass the test on other models of Tesla, all 18650 batteries used before will be replaced. Tesla's practice may lead a "21700 trend" in the world.
2、 Square lithium battery
Square lithium battery usually refers to aluminum shell or steel shell square battery. The popularity rate of square battery is very high in China. With the rise of automobile power battery in recent years, the contradiction between vehicle endurance mileage and battery capacity is increasingly prominent. Domestic power battery manufacturers mostly use aluminum shell square battery with high energy density, because the structure of square battery is relatively simple, unlike cylindrical battery Stainless steel with high strength is used as shell and accessories with explosion-proof safety valve, so the overall accessories are light in weight and relatively high in energy density. There are two different processes for square battery: winding and laminating.
But because the square lithium battery can be customized according to the size of the product, there are thousands of models on the market, and because there are too many models, the process is difficult to unify. There is no problem in the use of square battery in ordinary electronic products, but for industrial equipment products that need more than one series and parallel connection, it is better to use the standardized cylindrical lithium battery, so that the production process is guaranteed, and it is easier to find the replaceable battery in the future.
3、 Soft pack lithium battery
The key materials used in the soft pack lithium battery - positive material, negative material and diaphragm - have little difference from the traditional steel shell and aluminum shell lithium battery. The biggest difference lies in the soft pack material (aluminum plastic composite film), which is the most critical and technically difficult material in the soft pack lithium battery. Soft packaging materials are generally divided into three layers, namely, the outer resistance layer (generally the outer protection layer composed of nylon BOPA or PET), the impermeable layer (the middle layer of aluminum foil) and the inner layer (multi-functional high resistance layer).
The packaging materials and structure of soft battery make it have a series of advantages. The official account of electric power knows that, for example, the safety performance is good, the aluminum battery plastic package is used in the structure of the soft battery. When the safety problem occurs, the soft battery will generally blow up, but it will not explode like the steel shell or the aluminum shell electric core. 40% lighter, 20% lighter than aluminum shell lithium battery; small internal resistance, the internal resistance of soft battery is smaller than lithium battery, which can greatly reduce the battery's self consumption; good cycle performance, longer cycle life of soft battery, 100 cycle attenuation is 4% - 7% less than aluminum shell; flexible design, variable shape, can be thinner, customized according to the needs of customers, develop new cell models. The disadvantages of soft pack batteries are poor consistency, high cost and easy leakage. High cost can be solved by large-scale production, while leakage can be solved by improving the quality of aluminum plastic film.
4、 Comparative analysis of technical characteristics
1. Battery shape: the square lithium battery can be of any size, so it can't be compared with the cylindrical battery.
2. Multiplying characteristic: the technological limit of cylindrical lithium battery welding multipole, so the multiplying characteristic is slightly worse than that of square multipole
3. Discharge platform: the same positive material, negative material and electrolyte are used, so theoretically, the discharge platform is the same, but the internal resistance of the square battery is slightly superior, so the discharge platform is slightly higher.
4. Product quality: the process of cylindrical lithium battery is very mature, the probability of common secondary cutting defects is low, and the winding process is higher than the lamination process maturity and automation degree. At present, the lamination process is still in semi manual mode, so it has adverse impact on the quality of battery.
5. Electrode lug welding: the cylindrical lithium battery electrode lug is easier to be welded than the square lithium battery, and the square battery is easy to produce faulty welding, which affects the battery quality.
6. Pack group: the round battery is relatively easy to use, so the pack scheme is simple and has good heat dissipation effect. The square battery pack should solve the problem of heat dissipation.
7. Structure features: the chemical activity energy of square battery is poor at the corner, and the performance of long-term battery is obviously decreased.
In short, whether cylindrical, square or flexible battery, the reason why they can develop rapidly at present is that they have been well applied in their respective fields of application. Domestic power battery manufacturers are predicting that the square will become the mainstream of power battery, but the cylinder power battery application manufacturers represented by Tesla are constantly innovating in technology, the energy density of vehicle battery is constantly improving, and the performance expression of battery is more outstanding.